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Rotating crops

Careful management of irrigated potato crops over the long-term may help maintain crop productivity and nutrient availability within acceptable levels for agricultural production. This is the conclusion Agriculture and Agri-Food Canada (AAFC) researchers reached when they did a field experiment the year after completing a long-term irrigated potato rotation study in Brandon, Man.

During that initial 14-year rotation study, Ramona Mohr, a sustainable systems agronomist at AAFC, worked with a team of researchers to identify economically and environmentally sustainable rotations for irrigated potato.

From 1998 to 2011, Mohr and her colleagues grew six different rotations: potato-canola, potato-wheat, potato-canola-wheat, potato-oat-wheat, potato-wheat-canola-wheat and potato-canola (underseeded to alfalfa)-alfalfa-alfalfa, and looked at their effects on various factors including crop yield and quality, diseases and weeds, soil quality and economics.

“Generally accepted management practices were used during the 14-year rotation study,” Mohr says. “We soil tested on an annual basis and adjusted our fertilizer management practices accordingly.”

In the year after the rotation study ended, the researchers conducted a follow-up study to assess the impact preceding rotations had on phosphorus, potassium and micronutrient concentrations in the soil, as well as on soybean yield, quality and nutrient concentration.

For the follow-up study, the group picked a glyphosate-tolerant variety of soybean as an indicator crop to try to minimize possible confounding effects of the previous rotations with respect to disease, weeds and nitrogen.

It was a unique opportunity to study the relative effects preceding rotations have on crop productivity and nutrient status of the plant-soil system, given the limited information available for Western Canada.

“This rotation study was one of only two longer-term irrigated potato rotation studies conducted in Western Canada over the past couple of decades,” Mohr says.

Taking multiple soil samples from each rotation in the spring of 2012, the researchers were able to compare them with samples taken in 1997 at the beginning of the long-term study. They discovered that the soil nutrient levels and the yield and quality of the soybean crop were all typical of the region.

“We saw some differences in nutrient levels among the different rotations, but no substantial depletion or build up of nutrients, and no large differences in soybean yield among the rotations,” Mohr says.

Yield
Preceding rotations affected soybean yield to a limited degree. Soybean yield was six per cent higher following the potato-oat-wheat rotation than the potato-canola-wheat rotation, although the reason for this difference wasn’t clear.

“We also noticed when soybean followed potato, it had a slightly higher yield than following cereals or canola, we suspect, because of greater availability of water because the potato crop was irrigated and none of the other crops were,” Mohr says.

The samples also showed that where preceding rotations included alfalfa, seed protein increased and oil concentration decreased. The researchers believe the limited yield differences may have been due, in part, to the selection of soybean as an indicator crop. This likely minimized the differences among rotations arising from disease, weeds and nitrogen. Soybean had not been used in the rotation study and so was not susceptible to those diseases that had built up in the preceding rotations. Also, it is a nitrogen-fixing crop that could supply its own nitrogen. As well, by selecting a glyphosate-tolerant soybean, weeds could be effectively controlled regardless of the previous rotation.

Nutrients
For nutrients, the researchers looked at phosphorus and potassium in the top 15 centimetres of the surface soil and found they fell within the same general range of what they saw in 1997.

Soil test phosphorus was slightly higher except for the rotation that included alfalfa hay. That rotation saw a reduction of both phosphorus and potassium, which the researchers determined was likely a result of the high nutrient removal rates of alfalfa hay and their fertilizer management practices during the long-term study.

For example, Mohr says they applied phosphorus in the alfalfa establishment year, but not potassium because soil levels did not call for it. “When we saw a decline in soil potassium levels, we started applying potassium at that point.”

Soil test phosphorus levels were also found to be higher in the shorter rotations. “The two-year was greater than the three-year was greater than the four-year rotation,” Mohr says. Again, the researchers attribute this to their fertilizer management practices since they applied a higher rate because they broadcast the fertilizer in potato, adding more phosphorus than the potato crop removed.

“The more frequently we grew potatoes, the higher the soil test level we tended to see,” Mohr says. “Although we saw some difference in soil test levels, again they were within the range we see in agriculture fields.”

Micronutrients
Mohr and her colleagues also looked at micronutrients during their follow-up study. “The site had sufficient micronutrients for the crop so we didn’t need to add any fertilizer,” Mohr says, although certain of the fungicides applied to potato would have contained some copper or zinc. While the preceding rotation had minimal effects on soil copper and zinc levels, soybean established after the potato-canola-alfalfa rotations or directly following a potato crop contained comparatively higher seed copper and zinc concentrations.

This suggests, Mohr says, that including mycorrhizal crops, like potato and alfalfa, might have increased the availability of micronutrients to the following soybean crop. “The roots of potatoes and alfalfa form a relationship with mycorrhizal fungi in the soil, which may increase the availability of micronutrients. We didn’t measure mycorrhiza in this study, but we know that based on previous studies.”

Conclusion
The results of the follow-up study in 2012 do suggest that careful, long-term management of irrigated potato systems may help maintain crop productivity and nutrient availability within acceptable levels for agricultural production. However, Mohr stresses the impacts of disease, weeds and nitrogen fertility on crop growth may have been minimized because the researchers selected soybean as the indicator crop in this study.

“If producers were to grow other crop species, there might be a greater impact on crop productivity,” she says.

March 9, 2016  By Trudy Kelly Forsythe

Irrigating potato plots in the potato rotation study. Careful management of irrigated potato crops over the long-term may help maintain crop productivity and nutrient availability within acceptable levels for agricultural production. This is the conclusion Agriculture and Agri-Food Canada (AAFC) researchers reached when they did a field experiment the year after completing a long-term irrigated potato rotation study in Brandon